Hermetically sealed junction means



Jan. 12, 1960 c. L. WALLACE, JR., ETAL HERMETICALLY SEALED JUNCTIONMEANS Filed Oct. 8, 1958 F76 4-0 4/ 42 F/G 2 INVENTOR.

amen/cs 1.. mun- .12. EDWARD J. 0158040 BY Ms Mrs-Au ATIvEAIEKS UnitedStates Patent 2,921,245 HERMETICALLY SEALED JUNCTION IVIEANS Clarence L.Wallace, Jr., Topanga, and Edward J. Diel'rold, Palos Verdes Estates,Calif., assignors to International Rectifier Corporation, El Segundo,Calif., a corporation of California Application October 8, 1958, SerialNo. 766,032

15 Claims. (Cl. 317-234) This invention relates to rectifiers foralternating electric current.

It is well known that a rectifier containing a crystal of silicon whichis free from internal stresses, from structural defects, and fromsurface contamination has very valuable rectifying properties. Theseproperties can rarely be fully utilized because inherent in conventionalrectifiers is the likelihood of either contaminating the crystal inmanufacture, or stressing the crystal when the device is in use, in sucha manner that the rectifying capacity thereof is seriously reduced.

Contamination of the surface of a silicon crystal need not be great inorder to almost completely destroy the ability of the crystal to operateas a rectifier. Only very low concentrations, of molecular order, areenough to cause an excessive increase in reverse current. It has beenfound that even the most carefully conducted surface treatmentoperations tend to leave harmful impurities on the crystal, and that themore the crystal is treated the more impurities are deposited on thecrystal.

In addition because silicon rectifiers are particularly suited forrectifying heavy currents, they ordinarily are put in installationsWhere the crystal and its supporting structure are subjected to largetemperature fluctuations, and these fluctuations cause undesirablemechanical stresses. These stresses tend to destroy the rectifyingability of the device.

Patented Jan. 12, 1960 ice structure, thereby relieving lateralexpansion forces in the device. A flexible diaphragm interconnects thestud and the insulating ring, thereby relieving axial forces in thedevice.

A related feature of this invention resides in the method of assemblingthis rectifier, whereby the elements described above may be assembled ina furnace in stacked It is an object of this invention toprovide arectifier in which, in the entire course of its manufacture, the crystalsingle manufacturing step. During the time the crystal is exposed, it issurrounded and protected by a very pure protective atmosphere, whereby aminimal amount of impurities can reach the crystal. Furthermore, thematerials surrounding the crystal at the time of exposure are not of atype which would tend to contaminate it.

Still another object of the invention is to provide a structure in whichthe crystal is largely isolated from structural stresses which resultfrom temperature changes resulting from the passage of high currentsthrough the rectifier. Still another object of this invention is toprovide a rectifier which can be made of easily and inexpensivelyfabricated parts, and in which the manufacturing opera- :tions aresimple, and do not require complex or complicated procedures forminimizing contamination of the crystal.

This invention is carried out by installing a rectifying elementcomprising a semi-conductor such as crystal of silicon upon a base platewhich provides one terminal for relationship with bonding solders andthe like between them, and brazed together in the presence of aprotective atmosphere. By this means, the silicon crystal is exposed tooperations only once during the assembling process, and contamination isminimized by the protective atmosphere. Because the resulting structureis hermetically sealed by this operation, no additional contaminantsever reach the crystal.

The above and other features of this invention will be fully understoodfrom the following detailed description and the accompanying drawings,of which:

Fig. 1 is a side elevation partly in cut-away cross-section of arectifier according to the invention;

Fig. 2 is a cross-section taken at line 22 of Fig. 1;

Fig. 3 is a fragmentary cross-section showing another means ofinstalling a silicon wafer in the rectifier structure of Fig. 1;

Fig. 4 is a side elevation partly in cut-away cross-section showing twoof the rectifiers of the type shown inFig. I mounted in tandem andconnected to supporting structure; and

Fig. 5 shows another means for mounting a plurality of rectifiers of thetype shown in Fig. 1.

Fig. 1 illustrates a rectifier assembly 10 which includes a rectifierelement 11. The rectifier element is enclosed in a surrounding structureand is mounted to a base plate 12 which forms a part of the surroundingstructure.

The rectifier 11 includes a semi-conductor shown as a silicon wafer 13.This wafer consists essentially of pure intrinsic silicon, and is bondedto base plate 12 by a layer 14 of a solder which includes N-typeimpurities. N-type impurities are defined as metallic elements which arenegative compared to elements in the fourth column of the periodic tableof the elements, preferred elements for this purpose being selected fromthe fifth column, for example, arsenic, antimony and bismuth. The soldercomprises a low concentration of these relatively negative elements in acarrier. Thee carrier consists of a metal which has a relatively stableouter electron shell, examples being gold, copper and silver. As is wellknown, these carrier metals serve as a bonding agent between metalplates. Under heat treatment some of the N-type impurities will migrateinto the scilicon wafer so as to form a surface of N-type silicon with aboundary between what will become N-type silicon, and intrinsic silicon.A preferred N-type solder consists essentially of silver and a minoramount of antimony.

A layer 15 of a silicon-aluminum alloy serves to bond a metal disc 16 tothe wafer 13. Layer 15 is an alloy of silicon and aluminum. The aluminumis present to provide a P-type impurity for the silicon crystal. AP-type impurity is defined as a metal which is more positive than theelements in the fourth column of the periodic table of the elements.Aluminum is a P-type metal. Gallium, indium and thallium are severalother examples. These and alumium fall in the third column of theperiodic table of the elements which column contains the preferredP-type metals for this use. As is well known, under heat treatment, theP-type impurity migrates into the intrinsic silicon, and results in theformation of a surface layer of P-type silicon with a boundary betweenthe intrinsic silicon and the silicon with the P-type impurity.

As a result of the introduction of N-type impurities to one side of thecrystal, and P-type impurities to the other, a rectifying crystal havingthe physical composi- 3 tion of N-ty-pe, intrinsic, and P-type siliconin contiguity in that order is obtained, which is suitable forrectifying alternating electric current. For the purpose of definition,the rectifying element 11 is defined as comprising wafer 13 and the twosolder layers 14 and 15. .i

.The metaldisc 16-is preferably made of the;samematerial as the baseplate so as to minimize stresses on the wafer 13 which couldresultfrom.heatingtherectifier if the baseplate 12, disc 16, and a waferl3sattached to them were to have different .thermal coefiicients ofexpansion. Molybdenum is the .preferred metal for both the base plate 12and the disc 16, :because the thermal expansion of molybdenumcloselymatches .that of silicon. The rectifier element is not internallystressed to any significant degreeuwhen the rectifier is heated up withthis'arrangement, but it is pointed out that both :solder bonds aremoreflexible-than the plates and the disc on either side, so that shearstresses between the plate and disc and the crystal are largely taken upby the solder, and are not transmitted tothe crystal.

An elongated stud 17 is bonded to the disc 16 by a layer 18 of silversolder. This stud has. ascircularsection throughout its length, andterminates in a Tod-like upper section 20. An advantage of this shapeis;that it can easily be formed on a header by :a cheap fabricatronprocess.

A tubular element 21 is ring-shaped and:has.la U- shaped-radicalcross-section. This section'has a flatjcentral bight 22 (which isthereforena fiatannulus) and a pair of depending legs 23, 24. Itisevidentthat these two legs are both tubes which have their free endsdepending downwardly .from the :central bight. mThe lower ends of theselegs areattached by solder rings 25;. 26 to the base plate 12, with therectifier element attached to-the base plate, inside the tubular.element. 6

An insulator ring 27 iswplaced atop the tubular element 21. Itsupperandlower surfaces are metalized so that'the ring can receivesolderedconnections. wiThe ring is preferably made of a refractory material:which does not dissociate. at high temperature, such asaluminum oxide.Thelower end of the insulating ring is bonded by a solder ring 28 to thecentral bight of the tubular member 21.-.An upper solder ring 29 :is.provided for joining the outer-periphery of adiaphragm 30 to-the uppersurface ofthe ring.

Diaphragm 30 has acentral web 32 with an opening 33 at the center. Thereis a small turned-down flange 34 in the diaphragm adjacent the opening.which'provides withthe stud a depression-for receiving solder 35 thatbonds the diaphragm 30 tothestud.

A convenient means "for connecting the rectifier'into a circuit is to.provide a metal cup 36 which is inverted so that its rim 37 fits intoan annular-depression 38 in the diaphragm 30. Solder 39 may be used to'attac'h the rim of the cup to the diaphragm 30-. The -cu-p has anopening 40 at its central-portion for receiving a grommet 41 whichserves to attach a fiexible terminal "lead 42 to the cup. .A cooling fin43-ma'y be'bonded to the base plate by a layer 44 of solder, and the fincan then be used at the other term'inal of the-rectifier assembly,instead of the base plate, if desired.

In the construction shown inFig. l, the-order of polarity upward fromthe 'baseplate is N-intrinsic-P. It may be desired to-reversethis-polarity,:andprovide the order P-intrinsic-N. This is-achieved bysubstituting in place of the rectifier element 11 in Fig. 1, a rectifierelement 45, shown in Fig. 3.

. together. The. solder fort-he joints will be provided as discs orrings, as appropriate. Then graphite weights are applied to hold theassembly together, and the weighted assembly is transferred to an ovenhaving a hydrogen gas or inert gas atmosphere. The temperature iselevated so that the soldersy'rnelt to make their joints, and so.thatthe. Retype. and N-type impurities infuse into the silicon. Thestructure is then cooled and the rectifier is ready for use.

It will be noted that theatmosphere insidethering is that of thefurnace,- and :that after the solderwbonds have been hardened, the sameatmosphere remains in the hermetically sealed rectifier....This'atmosphere protected the wafer from oxidation during the heatingprocess, and afterass'ernbly, the surrounding structure isolates thewafer from any source of contamination. Furthermore, the metals withinthe structure are not the dissociative type, nor is the material of ring27, No contaminants are likely to reach the water at all.Therefore=thiszmeans oft-manufacture. exposes the wafer'only once, andthen itis not exposed .to any source of likely contamination.

The deviceshown-can be adapted to many :suitable structuralapplications. For example, inFig. 4,.a: rectifier 51 of the type shownin Fig. 1 has a terminal-lug 52 soldered thereto, instead of-thefiexible lead shown in'Fig. 1.

A second rectifier 53 of the'type shown in Fig. 1 has its cup solderedto a cooling fin 54.attached {to the upper rectifier 51, while its ownfin 55-is soldered to a hexagonal-headed stud 56 having a thread57zthereon. AsvshoWm-the stud can be threaded into .a-tapped:hole

58 in a bus bar 59. Persons skilled in the artwillapp'reciate-how stacksof-rectifiers such asthat shown in Fig--14 can be made up of any desiredlength -and how thevarious fins may be used for electrical terminals instructures such as bridge circuits and the like. The structure of Fig. 4can, forexample, be used as oneha-lf of a full-wave rectifier.

As an additional example, in Fig. 5 a pair ofrectifiers ofthe type shownin Fig. 1 are-illustrated connected in series. A spacer 60 holds the twofinsapart, whi1e a nut and bolt assembly 61 holds the fins in -tightassembly, so as to properly space and firmly mount the two rectifiers.The terminal lead 62 of a rectifier 63 -is connected by-a nut and boltassembly 64 to the fin 65 of another rectifier 66. The terminal lead 67of rectifier'66 provides one-terminal of series connection shownWhileanother lead 68 may be attached to the fin 69 ofrrectifier 63, tocomplete provisions for electricalconnections.

It will be observed that the silicon wafer is substantially protectedagainst stresses in all directions because of-the construction of the Adevice. With''- respect to lateral stresses, the solder layers oneachside-of the wafer of'semi-conductor material have lessresistancetoshear loadsthan the plates or other structures to which theybond the wafer. Therefore, when the base plate, silicon wafer, or studheats up and causes lateral expansion in theregion of the wafer,anydifierential movement will be taken up by the soft soldered bonds andlittle any force will beexerted directly upon the silicon wafer.

Lateral forces resulting from-the expansion of the baseplate-are alsotaken up by the tubular: member Rectifier element 45 includes a'silicon"wafer 46 which 21. It is well known that the end of a tube can beexpanded or contracted quite easily and without significantly distortingthe-tube ata distance significantly-removed from: the distorted end.Accordingly, when thelends of the tubular legs 23, '24 are expanded orcontracted; little if any of this movement is communicated to thenbight22 so that the-lateral expansion or-contra'ctionoftthe base plate. hasnosubstantial eifect on the over-allstructure, the tubular member actingto relieve them.

It-'is to be un'de'rstood that other typesofsiliconsjunctionsbouldw-have -beenw provided, such ascommon P-Njunctions wherea-crystaldopedwith one type of--impurity is contacted byanother type of impurity. The type of junction is completely variable,because the bonding materials can have either type of impurity therein,or even no impurity at all, and the wafer itself can be either intrinsicsilicon, or either type silicon.

With respect to difierential axial movements, the flexible diaphragm 30acts as a weak link in the system and can easily absorb any differentialmovement between the outer portion of the structure and the innerportion.

It will therefore be seen that this rectifier comprises an inexpensivestructure which is easily manufactured from conventional parts and whichinherently provides an easy means of manufacturing a clean,uncontaminated rectifier in which the full capabilities of asemi-conductor such as a silicon crystal in power rectification can beutilized.

This invention is not to be limited by the embodiments shown in thedrawings and described in the description, which are given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

What is claimed is:

1. A hermetically sealed rectifier comprising: a conductive base plate,a rectifying element conductively mounted on said base plate; a studconductively mounted on said rectifying element; a tube mounted to saidbase plate; an insulating ring mounted to said tube, said ring and tubesurrounding said rectifying element; and a diaphragm extending betweenand connected to the stud and to the insulating ring, the diaphragmmaking a fluid-tight seal between the ring and stud, the base plate,tube and ring completing, with the diaphragm, a fluidtight enclosure forthe rectifying element. a

2. A rectifier according to claim 1 in which the tube and ring aresubstantially circular cylinders.

3. A hermetically sealed current rectifier comprising: a base plate; arectifying element mounted to the base plate in conductive contacttherewith; a stud mounted to said rectifying element on the other sidethereof from the base plate; a channel member having a pair of legs anda central bight, the channel member being attached by its legs to thebase plate around the rectifying element, the bight being spaced fromthe base plate by the legs; an insulating ring mounted to the web of thechannel member on the other side thereof from the base plate andsurrounding the rectifying element; and a diaphragm extending betweenand connected .to the stud and to the insulating ring.

4. A hermetically sealed rectifier comprising: a base plate with a flatsurface; a semi-conductor wafer with a flat surface; a first metallicbond conductively mounting said wafer to the base plate insurface-to-surface relationship, the metallic bond having a shearstrength less than that of the base plate and wafer; a tubular memberattached to said base plate, surrounding the wafer; an insulating ringmounted to the tubular member on the opposite side thereof from the baseplate; a stud conductively mounted to the wafer on the other sidethereof from the base plate; and a flexible diaphragm interconnectingthe insulating ring and the stud.

5. Apparatus according to claim 4 in which the insulating ring is madeof refractory material, and the remainder of the rectifier is made ofmetal.

6. Apparatus according to claim 4 in which the tutular member has aU-shaped cross-section formed by a central bight and two depending legs,the legs being tubular, and attached to the base plate.

7. Apparatus according to claim 6 in which the insulat ing ring is madeof refractory material, and the remainder of the rectifier is made ofmetal.

8. Apparatus according to claim 4 in which the first metallic bondincludes an N-type impurity, and in which the stud is mounted to thewafer by a metal plate to which it is bonded, a second metallic bondbonding said metal plate to the other side of the wafer and including aP-type impuritv.

9. Apparatus according to claim 4 in which the first metallic bondincludes an N-type impurity, said first bond mounting the wafer to thebase plate by a first metal plate, said first plate being conductivelybonded to said base plate, and in which the stud is mounted to the waferby a second metal plate; a second metallic bond bonding said secondmetal plate to the other side of the wafer, and including a P-typeimpurity.

10. A hermetically sealed rectifier comprising: a. base plate with aflat surface; a semi-conductor wafer with a fiat surface; a firstmetallic bond conductively mounting said rectifying element to the baseplate in surface-tosurface relationship, the first metallic bond havinga shear strength less than that of the base plate and wafer; atubular-member attached to said base plate, said tubular member beinggenerally cylindrical and having in crosssection a central bight and twotubular legs extending from the same side of said bight, said tubularlegs being attached to said base plate; an insulating ring mounted tothe bight of the tubular member; a stud conductively mounted to therectifying element on the other side of the base plate; and a flexiblediaphragm interconnecting the insulating ring and the stud and forming afluid-type barrier therebetween; said stud being mounted to therectifying element by means of a metal plate interposed therebetweenwhich is bonded to the stud and to the wafer; and a second metallic bondbonding the metal disc to the wafer, said base plate, tubular member,insulating ring, diaphragm, and stud being continuous structures,continuously joined to form a fluid-tight structure.

11. Apparatus according to claim 10 in which the first metallic bondincludes an N-type impurity and in which the second metallic bondincludes a P-type impurity.

12. Apparatus according to claim 10 in which the first metallic bondincludes a P-type impurity and the second metallic bond includes anN-type impurity.

13. Apparatus according to claim 10 in which the outer periphery of thediaphragm has an annular groove therein, and in which a cup having a rimis inverted over the diaphragm with its rim conductively bonded in theannular groove whereby a first electrical connection can be made throughthe cup, diaphragm, stud and metal plate to the rectifying element, thebase plate providing a second connection.

14. Apparatus according to claim 13 in which a terminal lead isconnected to the cup.

15. Apparatus according to claim 13 in which a cooling fin is attachedto the base plate in heat-transfer relationship therewith.

References Cited in the file of this patent UNITED STATES PATENTS2,756,374 Colleran et al July 24, 1956 2,861,226 Lootens Nov. 18, 19582,864,980 Mueller et a1. Dec. 16, 1958

